Dielectric ceramic composition containing ZnO-B2 O3 -SiO2 glass

ABSTRACT

A dielectric ceramic composition is disclosed which consists essentially of: a main ceramic composition containing barium oxide, titanium oxide, rare earth oxide and bismuth oxide as major components, which composition is represented by xBaO·yTiO 2  ·Z[(1-a)RE 2  O 3  ·aBi 2  O 3  ] where RE represents at least one rare earth metal, 0.10≦x≦0.20, 0.60≦y≦0.75, 0.10 ≦z≦0.25, x+y+z=1 and 0&lt;a≦0.3; and a glass composition composed of ZnO, B 2  O 3  and SiO 2 , which is represented by k(wt.%)ZnO·m(wt.%)B 2  O 3  ·n(wt.%)SiO 2  where 30 ≦k≦85, 5≦m≦50, 2≦n≦40, k+m+n=100. The glass composition is contained in an amount of at least 0.1 part by weight, but not more than (18-62.5a) parts by weight (where 0&lt;a≦0.2) or not more than 5.5 parts by weight (where 0.2&lt;a≦0.3), per 100 parts by weight of the main ceramic composition. The main ceramic composition may have another combination of major components. Also disclosed are a method of preparing the dielectric ceramic composition as described above, a dielectric resonator and filter using such a dielectric ceramic composition, and a method of producing such a dielectric resonator or filter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to a dielectric ceramiccomposition suited for low temperature firing, and a method of preparingthe same In particular, the invention is concerned with a dielectricceramic composition for microwave applications, which can be fired at arelatively low temperature, and which is suitably used for a dielectricresonator having internal conductive layers, of a stripline type filter,for example, and with a method of preparing such a dielectric ceramiccomposition The present invention is also concerned with a dielectricresonator obtained by using such a dielectric ceramic composition, adielectric filter including a plurality of such dielectric resonators,and with a method of producing the dielectric resonator or dielectricfilter.

2. Discussion of the Prior Art

In a modern microwave telecommunication system such as a portable orautomobile telephone system, there is widely used a coaxial typedielectric filter using a ceramic composition having a high dielectricconstant. The coaxial type dielectric filter has a plurality of coaxialtype resonators connected to each other. Each resonator is a cylindricaldielectric block which has inside and outside conductors formed on innerand outer circumferential surfaces of the block, respectively. This typeof dielectric filter has a limitation in reducing the size and thicknessthereof due to its construction. In view of this, there is proposed astripline type filter of a tri-plate structure, which incorporatesinternal conductive layers or strips within a dielectric substrate. Inthis stripline type filter, a patterned array of conductors in the formof strips are integrally embedded in the dielectric substrate so as toprovide a plurality of resonators. The thus constructed stripline typefilter is comparatively compact and thin.

In fabricating such a stripline type dielectric filter having theinternal conductive layers or strips as described above, a dielectricceramic composition must be co-fired with the internal conductivelayers. Since known dielectric ceramic compositions have a considerablyhigh firing temperature, there is a limit to conductive materials whichcan be used for the internal conductive layers, thus making it difficultto employ an Ag-contained material having a relatively low conductivityresistance For instance, the firing temperature of the dielectricceramic composition must not exceed 1000° C. when Ag-Pd or Ag-Pt alloysare used for the internal conductive layers, and, in particular, must becontrolled to be around 900° C. when the conductive layers are formedsolely of Ag having a low conductivity resistance. To meet the needs, itis required to provide a dielectric ceramic composition which can befired at a sufficiently low temperature while assuring excellentmicrowave characteristics.

Among various dielectric ceramic compositions which have been proposed,a dielectric ceramic composition which contains oxides of Ba, Ti, RE(rare earth metals) and Bi is known as having a high specific dielectricconstant, a large unloaded Q, and a small temperature coefficient of theresonance frequency. This composition, however, has a problem in itsfiring temperature as high as 1300°-1400° C., and various attempts havebeen made to reduce the firing temperature, by the addition of oxides ofPb, for example.

An example of such dielectric ceramic composition is disclosed in U.S.Pat. No. 3,811,937, wherein a calcined mixture of BaO, TiO₂ and a rareearth oxide is blended with 8 to 30% by weight of a glass formulationcontaining CdO, PbO and Bi₂ O₃. The thus prepared composition is firedat a temperature between about 982° C. and 1150° C. Another example ofdielectric ceramic composition as disclosed in JP-A-59-214105 containsBaO, TiO₂ and Nd₂ O₃ as major components, which are mixed with powdersof PbO, Bi₂ O₃, SiO₂ and ZnO. This composition is fired at a temperaturebetween 1050° C. and 1100° C. A further example of composition asdisclosed in JP-B2-4-16884 contains BaTiO₃, Nd₂ O₃, TiO₂ and Bi₂ O₃ asmajor components, to which Pb₃ O₄, B₂ O₃, SiO₂ and ZnO are added inrespective suitable amounts. This composition is fired at a temperaturebetween 1000° C. and 1050° C. A still further example of dielectricceramic composition as disclosed in JP-A-2-44609 contains BaTiO₃, Nd₂O₃, TiO₂, Bi₂ O₃ and Pb₃ O₄ as major components, to which 2CaO·3B₂ O₃,SiO₂ and ZnO are added This composition is fired at a temperaturebetween 1000° C. and 1050° C.

The known dielectric ceramic compositions as described above, which canbe fired at a relatively low temperature, still has a firing temperatureof around 1000° C. or higher, and thus cannot be used with internalconductors formed solely of Ag having a low conductivity resistance, oralloys consisting principally of Ag. In fact, these compositions can beused only with internal conductors formed of Ag-Pd alloys including arelatively high content of Pd having a large conductivity resistance.The known low firing temperature dielectric ceramic compositions, whichcontain a relatively large amount of Pb oxides, also have some problemupon handling thereof, in view of the toxicity of the Pb oxides.

While some known techniques are available for lowering the firingtemperature of a dielectric ceramic composition down to around 1000° C.,there have been unknown such techniques as permitting the firing at atemperature lower than the melting point of Ag, i.e., 962° C., desirablyat 950° C. or lower, more desirably at around 900° C.

SUMMARY OF THE INVENTION

The present invention was developed in the light of the prior artsituations described above. It is therefore a first object of theinvention to provide a low temperature firing dielectric ceramiccomposition which gives dielectric ceramics having a relatively highspecific dielectric constant, a relatively large unloaded Q and arelatively small temperature coefficient of the resonance frequency, andwhich can be fired or sintered at a temperature of not higher than 962°C. (the melting point of Ag), preferably at around 900° C., withoutrequiring the addition of a large amount of Pb oxides.

It is a second object of the invention to provide a method of preparingsuch a dielectric ceramic composition as described above.

It is a third object of the invention to provide a dielectric resonatorobtained by using such a dielectric ceramic composition as describedabove. It is a fourth object of the invention to provide a dielectricfilter including such dielectric resonators. It is a fifth object of theinvention to provide a method of producing such a dielectric resonatoror dielectric filter.

Having made various attempts and analyses to accomplish the firstobject, the inventors found that a dielectric ceramic compositioncontaining BaO, TiO₂, RE₂ O₃ and Bi₂ O₃ as major components can be firedat a significantly lowered temperature, while assuring excellentdielectric properties, by adding a relatively small amount of suitableglass containing ZnO, B₂ O₃ and SiO₂ to the major components.

Namely, the first object may be accomplished according to a first aspectof the present invention, which provides a dielectric ceramiccomposition which consists essentially of: a main ceramic compositioncontaining barium oxide, titanium oxide, rare earth oxide and bismuthoxide as major components, which composition is represented byxBaO·yTiO₂ ·z[(1-a)RE₂ O₃ ·aBi₂ O₃ ] where RE represents at least onerare earth metal, 0.10≦x≦0.20, 0.60≦y≦0.75, 0.10≦z≦0.25, x+y+z=1 and0<a≦0.3; and a glass composition composed of ZnO, B₂ O₃ and SiO₂, whichis represented by k(wt.%)ZnO·m(wt.%)B₂ O₃ ·n(wt.%)SiO₂ where 30≦k85,5≦m≦50, 2≦n≦40, k+m+n=100, the glass composition being contained in anamount of at least 0.1 part by weight, but not more than (18-62.5a)parts by weight (where 0<a≦0.2) or not more than 5.5 parts by weight(where 0.2<a≦0.3), per 100 parts by weight of the main ceramiccomposition. This dielectric ceramic composition may be fired at arelatively low temperature, assuring excellent microwavecharacteristics.

According to one feature of the above aspect of the invention, the mainceramic composition further contains not more than 2.5 parts by weightof alumina per 100 parts by weight of a total amount of the majorcomponents. In this case, the dielectric ceramic composition may befired at a relatively low temperature, assuring a further improvedunloaded Q.

According to another feature of the above aspect of the invention, themain ceramic composition further contains not more than 3 parts byweight of manganese oxide per 100 parts by weight of a total amount ofthe major components In this case, the dielectric ceramic compositioncan be co-fired with a conductor material at a relatively lowtemperature, assuring a sufficiently high specific dielectric constant,a further improved unloaded Q and a significantly reduced temperaturecoefficient of the resonance frequency.

The second object indicated above may be attained according to a secondaspect of the invention, which provides a method of preparing thedielectric ceramic composition as described above, comprising the stepsof: excluding at least a part of bismuth oxide from starting materialswhich give the main ceramic composition, calcining and pulverizing amixture of the starting materials to provide a calcined ceramic powder;and blending the calcined ceramic powder with the at least a part ofbismuth oxide and the glass composition composed of ZnO, B₂ O₃ and SiO₂.According to this method, the firing temperature of the dielectricceramic composition obtained can be considerably lowered.

According to one feature of the second aspect of the invention, themixture of the starting materials is calcined at 1050° C. or higher toprovide a calcined mixture. Further, the calcined mixture may be finelypulverized so that the calcined ceramic powder has an average grain sizeof not larger than 0.8μm. In this case, the firing temperature of thedielectric ceramic composition obtained is remarkably lowered, while thespecific dielectric constant and unloaded Q thereof are significantlyimproved or increased.

In the dielectric ceramic compositions according to the above-indicatedfirst aspect of the invention, if the content of BaO is smaller than 10mole % (x<0.10), the specific dielectric constant of the obtaineddielectric ceramics is undesirably lowered. If the BaO content exceeds20 mole % (x>0.20), the dielectric ceramics suffer from an excessivelylarge temperature coefficient of the resonance frequency. If the TiO₂content of the dielectric ceramic compositions is smaller than 60 mole %(y<0.60), it becomes difficult to sinter the ceramic composition to givea sintered ceramic body having a sufficiently high density. If the TiO₂content exceeds 75 mole % (y>0.75), the temperature coefficient of theresonance frequency undesirably goes highly positive.

If the total content of RE₂ O₃ and Bi₂ O₃, i.e., the above-indicatedterm [(1-a)RE₂ O₃ ·aBi₂ O₃ ] is smaller than 10 mole % (z>0.10), thetemperature coefficient of the resonance frequency undesirably goeshighly positive. If the total content of RE₂ O₃ and Bi₂ O₃ exceeds 25mole % (z> 0.25), the sinterability of the composition is deteriorated,resulting in reduction in the specific dielectric constant.

According to another feature of the first aspect of the presentinvention, RE (rare earth metal) present in RE₂ O₃ is selected from Nd,Sm, La, Ce, Pr and other rare earth metals. Preferably, RE consists ofNd, or a combination of Nd and Sm and/or La When Nd is used incombination with Sm and/or La, the temperature coefficient of theresonance frequency can be appropriately controlled while assuring highspecific dielectric constant and large unloaded Q. In this case,however, it is desirable that the proportion of Sm and/or La to thewhole RE is equal to or smaller than 20 mole %. If it exceeds 20 mole %,the temperature coefficient of the resonance frequency is varied to agreat extent in the positive or negative direction. 0n the other hand,the amount of Ce or Pr that is introduced into the ceramic compositionas RE is determined as converted into trivalent atoms.

When a portion of RE₂ O₃ is substituted by Bi₂ O₃, the specificdielectric constant can be increased while the temperature coefficientof the resonance frequency can be reduced. It is particularly desirableto substitute at least 5 mole % (a≧0.05) of RE₂ O₃, so as to achievesatisfactory effects due to the substitution by Bi₂ O₃. Since thetemperature coefficient comes to undesirably increase if thesubstituting amount of Bi₂ O₃ reaches 15-20 mole % or more, and theunloaded Q decreases with an increase in the substituting amount of Bi₂O₃, the practical substituting amount of Bi₂ O₃ is appropriately kept tobe not more than 30 mole % (a≦0.30).

The dielectric ceramic compositions according to the above aspect of theinvention consists essentially of the major ceramic compositioncontaining barium oxide, titanium oxide, rare earth oxide and bismuthoxide in the above-indicated proportion, and a suitable amount of theglass composition as described below However, suitable metal oxide, suchas alumina, iron oxide, manganese oxide, chromium oxide, zinc oxide,stannic oxide (tin oxide), or zirconia, may be added to the main ceramiccomposition as described above, in an attempt to improve the unloaded Qand suitably adjust or control the temperature coefficient of theresonance frequency. In particular, the addition of alumina to the aboveceramic composition is considerably effective to further improve theunloaded Q. On the other hand, manganese oxide functions to preventreduction of the main ceramic composition, and therefore favorablypermits co-firing thereof with copper conductors, for example, in anitrogen atmosphere. The addition of the manganese oxide is alsoeffective to improve the unloaded Q while maintaining a sufficientlyhigh dielectric constant, and decreases the temperature coefficient ofthe resonance frequency. The alumina and the manganese oxide arerespectively added to the main ceramic composition consisting of bariumoxide, titanium oxide, rare earth oxide and bismuth oxide, in amounts ofup to 2.5 and up to 3 parts by weight, respectively, per 100 parts byweight of the above main ceramic composition, preferably, in amounts ofup to 2 parts by weight.

The glass composition of the ZnO--B₂ O₃ --SiO₂ system in the presentdielectric ceramic composition consists of 30-85% by weight of zincoxide (ZnO), 5-50% by weight of boric oxide (B₂ O₃), and 2-40% by weightof silicon oxide (SiO₂). Namely, when the contents of zinc oxide, boricoxide and silicon oxide are respectively represented by k wt. %, m wt. %and n wt. %, it is necessary to satisfy the following formulas 30≦k≦85,5≦m≦50, 2≦n≦40, and k+m+n =100. The addition of ZnO, B₂ O₃ and SiO₂ inthe form of glass to the main ceramic composition is effective to lowerthe firing temperature of the dielectric ceramic composition obtained.Of the constituent elements of the glass, SiO₂ is consideredparticularly important in forming glass. Namely, the SiO₂ element doesnot only facilitate vitrification of the composition, but also stabilizethe glass obtained.

If the content (k) of ZnO is smaller than 30% by weight, the glasscomposition is less likely to be vitrified to form a desired glass. Ifthe ZnO content (k) exceeds 85% by weight, the glass composition is hardto be vitrified, and the firing temperature of the dielectric ceramiccomposition prepared is undesirably increased. If the content (m) of B₂O₃ is smaller than 5% by weight, the vitrification of the glasscomposition becomes difficult, and the firing temperature of thedielectric ceramic composition is undesirably increased. If the B₂ O₃content (m) exceeds 50% by weight, the dielectric ceramics obtainedsuffers from reduction in the unloaded Q. If the SiO₂ content (n)exceeds 40% by weight, the glass composition is hard to be vitrified,and the firing temperature of the dielectric ceramic compositionprepared is increased. The inclusion of SiO₂ in the glass compositionresults in easy vitrification of the composition, whereby the dielectricceramic composition containing the thus formed glass can be fired at asignificantly lowered temperature, assuring excellent microwavecharacteristics.

The preferred composition range of the ZnO--B₂ O₃ --SiO₂ glass is suchthat the ZnO content (k) is in a range of 40-75 wt. %, the B₂ O₃ content(m) is in a range of 10-40 wt. %, and the SiO₂ content (n) is in a rangeof 5-30 wt. %. While the thus prepared glass composition may be allowedto include impurities such as various metal oxides, it is to begenerally understood that the content of such impurities in the glasscomposition must be kept to be not more than 10% by weight. Further, theZnO--B₂ O₃ --SiO₂ glass used for the present invention may not beentirely uniformly vitrified. Namely, the ZnO--B₂ O₃ --SiO₂ glass mayhave different phases from portion to portion, or may include a portionformed of unvitrified materials, or may be partially crystallized, aslong as the glass is substantially in a vitrified condition.

The ZnO--B₂ O₃ --SiO₂ glass having the above-described composition iscontained as a secondary component in the present dielectric ceramiccomposition, in an amount of at least 0.1 part by weight, but not morethan (18-62.5a) parts by weight (where 0<a≦0.2) or not more than 5.5parts by weight (where 0.2<a≦0.3), per 100 parts by weight of the mainceramic composition as described above, whereby the firing temperatureof the dielectric ceramic composition obtained can be lowered to 962° C.or lower, desirably 950° C. or lower, more desirably around 900° C. Ifthe content of the glass is smaller than 0.1 part by weight, theaddition of the glass component does not yield a satisfactory effect tolowering the firing temperature. If the content of the glass exceeds(18-62.5a) parts by weight where 0<a≦0.2), or exceeds 5.5 parts byweight where 0.2<a≦0.3, the dielectric ceramics obtained have anexcessively reduced unloaded Q, and are thus not suitable for practicaluse.

The dielectric ceramic composition according to the first aspect of thepresent invention is prepared by blending the main ceramic compositionas described above, with the above ZnO--B₂ O₃ --SiO₂ glass composition.Prior to the addition (blending) of the ZnO--B₂ O₃ --SiO₂ glass, amixture of starting materials giving the main ceramic composition iscalcined and then pulverized. When the calcining temperature is 900° C.or higher, the firing temperature of the dielectric ceramic compositionis reduced while the specific dielectric constant and unloaded Q areincreased, due to the relatively high calcining temperature. Preferably,the calcining of the main ceramic composition is carried out at atemperature of 1050° C. or higher, so as to most effectively yield theabove effects or advantages. If the calcining temperature exceeds 1350°C., however, the calcined mass is likely to excessively harden after thecalcining process, causing some problem in handling thereof Therefore,the calcining temperature is preferably held between 1100° C. and 1300°C

In pulverizing the thus calcined mass, the smaller the average grainsize of a ceramic powder obtained as a result of pulverization, thelower the firing temperature of the dielectric ceramic compositionobtained, resulting in increases in the specific dielectric constant andunloaded Q. Accordingly, according to the present invention, thecalcined mass is pulverized to achieve the average grain size of notlarger than 0.8 μm. If the average grain size of the ceramic powder issmaller than 0.1 μm, however, the formability of the dielectric ceramiccomposition obtained is deteriorated, making it difficult to form a tapeby an ordinary doctor blade technique, for example. Therefore, theaverage grain size of the ceramic powder is desirably controlled towithin a range of about 0.1˜0.8 μm. Generally, such a small grain sizeof the ceramic powder is measured by a laser diffraction and scatteringmethod.

In preparing the dielectric ceramic compositions according to the abovefirst aspect of the invention, at least a part of bismuth oxide as onemajor component of the main ceramic composition may be added later tothe main ceramic composition, together with the ZnO--B₂ O₃ --SiO₂ glasscomponent. Namely, at least a part of bismuth oxide is excluded fromstarting materials which will give the main ceramic composition asdescribed above, and a mixture of the starting materials is calcined andpulverized. The calcined mixture thus pulverized is blended with thebismuth oxide excluded, together with the ZnO--B₂ O₃ --SiO₂ glasscomponent. Consequently, the firing temperature of the dielectricceramic composition thus obtained can be effectively lowered, i.e., thehigher the proportion of the bismuth oxide added later, the lower thefiring temperature. In this regard, the proportion of the bismuth oxideadded later is desirably kept to be not more than 50% by weight, sincethe unloaded Q decreases if the proportion exceeds 50% by weight.Namely, up to 50% by weight of bismuth oxide may be added later to themain ceramic composition.

The above-indicated first object of the present invention may be alsoattained according to a third aspect of the present invention, whileprovides a dielectric ceramic composition which consists essentially of:a main ceramic composition containing barium oxide, titanium oxide andrare earth oxide as major components, which composition is representedby xBaO·yTiO₂ ·zRE₂ O₃ where RE represents at least one rare earthmetal, 0.10≦x≦0.20, 0.60≦y ≦0.75, 0.10≦z≦0.25, and x+y+z1; and a glasscomposition composed of ZnO, B₂ O₃ and SiO₂, which is represented byk(wt. %)ZnO·m(wt. %)B₂ O₃ ·n(wt. %)SiO₂ where 30≦k≦85, 5≦m≦50, 2≦n≦40,k+m+n=100, the glass composition being contained in an amount of 0.1˜18parts by weight per 100 parts by weight of the main ceramic composition.

The main ceramic composition may further contain not more than 2.5 partsby weight of alumina per 100 parts by weight of a total amount of themajor components. In this case, the dielectric ceramic compositionobtained may be fired at a relatively low temperature, assuring asignificantly improved unloaded Q.

Alternatively, the main ceramic composition may further contains notmore than 3 parts by weight of manganese oxide per 100 parts by weightof a total amount of the major components. In this case, the dielectricceramic composition can be co-fired with copper conductors, for example,in a nitrogen atmosphere, assuring a further improved unloaded Q and asignificantly reduced temperature coefficient of the resonancefrequency.

In the dielectric ceramic composition as defined just above, thecontents of BaO and TiO₂ are held within the respective specifiedranges, for the same reasons as given above with respect to thedielectric ceramic composition according to the first aspect of theinvention.

If the content of RE₂ O₃ in the above dielectric ceramic composition issmaller than 10 mole % (z>0.10), the temperature coefficient of theresonance frequency undesirably goes highly positive. If the content ofRE₂ O₃ exceeds 25 mole % (z>0.25), the sinterability of the compositionis deteriorated, resulting in reduction in the specific dielectricconstant.

Further, RE present in RE₂ O₃ may be selected from the above-indicatedrare earth metals referred to above with respect to the dielectricceramic composition according to the first aspect of the invention. WhenNd is used in combination with Sm and/or La, it is desirable that theproportion of Sm and/or La to the whole RE is equal to or smaller than40 mole %. If it exceeds 40 mole %, the temperature coefficient of theresonance frequency is varied to a great extent in the positive ornegative direction.

As described above with respect to the first aspect of the invention,suitable metal oxide, such as alumina, iron oxide, manganese oxide,chromium oxide, zinc oxide, stannic oxide (tin oxide), or zirconia, maybe added in a suitable proportion to the main ceramic composition in theBaO--TiO₂ --RE₂ O₃ system, for the same purposes as described above.

In the present dielectric ceramic composition, the contents of ZnO, B₂O₃ and SiO₂ in the glass composition are held within the respectivespecified ranges, for the same reasons as given above with respect tothe dielectric ceramic composition according to the first aspect of theinvention.

The ZnO--B₂ O₃ --SiO₂ glass having the above-described composition iscontained as a secondary component in the present dielectric ceramiccomposition, in an amount of 0.1 ˜18 parts by weight per 100 parts byweight of the main ceramic composition as described above, whereby thefiring temperature of the dielectric ceramic composition obtained can belowered to 962° C. (melting point of Ag) or lower, desirably 950° C. orlower, more desirably around 900° C. If the content of the glass issmaller than 0.1 part by weight, the addition of the glass componentdoes not yield a satisfactory effect of lowering the firing temperature.If the content of the glass exceeds 18 parts by weight, the dielectricceramics obtained have an excessively reduced unloaded Q, and are thusnot suitable for practical use.

In preparing the dielectric ceramic composition as described above, themain ceramic composition of the BaO--TiO₂ --RE₂ O₃ system is preferablycalcined at 1050° C. or higher, more preferably, at a temperaturebetween 1100° C. and 1300° C.

The thus calcined mass of the main ceramic composition is pulverized toachieve the average grain size of not larger than 0.8 μm. Preferably,the average grain size of the ceramic powder is controlled to within arange of about 0.1˜0.8 μm.

The above-described first object may be also attained according to afourth aspect of the present invention, which provides a dielectricceramic composition which consists essentially of: a main ceramiccomposition containing barium oxide, strontium oxide, calcium oxide,titanium oxide, rare earth oxide and bismuth oxide as major components,which composition is represented by x[(1-c-d)BaO·cSrO·dCaO]·yTiO₂·z[(1-e)RE₂ O₃ ·eBi₂ O₃ ] where RE represents at least one rare earthmetal, 0.10≦x≦0.20, 0.60≦y≦0.75, 0.10≦z≦0.25, x+y+z=1, 0≦c≦0.40,0≦d≦0.20, 0≦e≦0.30 and 0<c+d; and a glass composition composed of ZnO,B₂ O₃ and SiO₂, which is represented by k(wt. %)ZnO·m(wt. %)B₂ O₃ ·n(wt.%)SiO₂ where 30≦k≦85, 5≦m≦50, 2≦n≦40, k+m+n=100, the glass compositionbeing contained in an amount of at least 0.1 part by weight, but notmore than (18-62.5e) parts by weight (where 0< e≦0.2) or not more than5.5 parts by weight (where 0.2<e≦0.3), per 100 parts by weight of themain ceramic composition.

In the dielectric ceramic composition as defined just above, the term"x[(1-c-d)BaO·cSrO·dCaO]" of the main ceramic composition indicates thata portion of BaO as one major component is substituted by SrO and/orCaO. If the total content of BaO, SrO and CaO is smaller than 10 mole %(x<0.10), the specific dielectric constant of the obtained dielectricceramics is undesirably lowered. If the total content exceeds 20 mole %(x>0.20), the dielectric ceramics suffer from an excessively largetemperature coefficient of the resonance frequency.

The partial substitution of BaO is carried out by using at least one ofSrO and CaO. When a portion of BaO is substituted by SrO, for example,the unloaded Q may be improved and the temperature coefficient of theresonance frequency may be reduced due to the substitution, whilemaintaining a high specific dielectric constant If the proportion of thesubstitution by SrO exceeds 0.40 (c>0.40), however, the dielectricconstant and unloaded Q are both deteriorated. When a portion of BaO issubstituted by CaO, the temperature coefficient of the resonancefrequency may be increased while maintaining high specific dielectricconstant and large unloaded Q. If the proportion of the substitution byCaO exceeds 0.20 (d<0.20), however, the unloaded Q is rapidlydeteriorated. Accordingly, the temperature coefficient of the resonancefrequency may be advantageously controlled by the addition of suitableamounts of SrO and/or CaO.

In the dielectric ceramic composition as defined just above, the content(mole %) of TiO₂ is held within the specified range, i.e., 0.60≦y≦0.75,for the same reason as given above with respect to the dielectricceramic composition according to the first aspect of the invention.

The total amount of RE₂ O₃ and Bi₂ O₃, i.e., the above-indicated term[(1-e)RE₂ O₃ and eBi₂ O₃ ], is also held within the specified rangeindicated above, i.e., 0.10≦z≦0.25, for the same reason as given abovewith respect to the total amount of RE₂ O₃ and Bi₂ O₃ in the dielectricceramic composition according to the first aspect of the invention.

Further, RE present in RE₂ O₃ may be selected from the rare earth metalsindicated above with respect to the dielectric ceramic compositionsaccording to the first aspect of the invention. When Nd is used incombination with Sm and/or La, it is desirable that the proportion of Smand/or La to the whole RE is equal to or smaller than 20 mole %. If itexceeds 20 mole %, the temperature coefficient of the resonancefrequency is varied to a great extent in the positive or negativedirection

The substitution of RE₂ O₃ by Bi₂ O₃ is effected such that thesubstituting amount of Bi₂ O₃ is kept within the specified rangeindicated above, i.e., 0≦e≦0.30, preferably, 0.05≦e≦0.30, for the samereason as given above with respect to the dielectric ceramic compositionaccording to the first aspect of the invention.

As described above with respect to the first aspect of the invention,suitable metal oxide, such as alumina, iron oxide, manganese oxide,chromium oxide, zinc oxide, stannic oxide (tin oxide), or zirconia, maybe added to the above main ceramic composition in theBaO--SrO--CaO--TiO₂ --RE₂ O₃ --Bi₂ O₃ system, in order to improve theunloaded Q and suitably adjust or control the temperature coefficient ofthe resonance frequency.

In the present dielectric ceramic composition, the contents of ZnO, B₂O₃ and SiO₂ in the glass composition are held within the respectivespecified ranges, for the same reasons as given above with respect tothe dielectric ceramic composition according to the first aspect of theinvention.

As described above, the preferred composition range of the ZnO--B₂ O₃--SiO₂ glass is such that the ZnO content (k) is in a range of 40-75 wt.%, the B₂ O₃ content (m) is in a range of 10-40 wt. %, and the SiO₂content (n) is in a range of 5-30 wt. %.

The content of the ZnO--B₂ O₃ --SiO₂ glass having the above-describedcomposition in the present dielectric ceramic composition is kept withinthe specified range indicated above, for the same reasons as given abovewith respect to the dielectric ceramic composition according to thefirst aspect of the present invention. Preferably, the ZnO--B₂ O₃ --SiO₂glass is contained in the present dielectric ceramic composition, in anamount of 1-3 parts by weight, per 100 parts by weight of the mainceramic composition.

In preparing the present dielectric ceramic composition, the mainceramic composition as described above is preferably calcined at 1050°C. or higher, more preferably, at a temperature between 1100° C. and1300° C.

The thus calcined mass of the main ceramic composition is pulverized toachieve the average grain size of not larger than 0.8 μm. Preferably,the average grain size of the ceramic powder obtained is controlled towithin a range of about 0.1˜0.8 μm.

In preparing the present dielectric ceramic composition, at least a partof bismuth oxide in the main ceramic composition may be added later tothe main ceramic composition, along with the ZnO--B₂ O₃ --SiO₂ glasscomponent. The proportion of the bismuth oxide added later is desirablykept to be not more than 50% by weight.

The third object of the invention indicated above may be achievedaccording to a fifth aspect of the invention, which provides adielectric resonator comprising: a dielectric ceramic obtained by firinga dielectric ceramic composition according to any one of the first,second and third aspects of the invention which have been describedabove; and a conductor pattern which is formed by co-firing with thedielectric ceramic so that the conductor pattern is embedded in thedielectric ceramic.

The fourth object of the invention indicated above may be achievedaccording to a sixth aspect of the invention, which provides adielectric filter including a plurality of dielectric resonatorsaccording to the above fifth aspect of the invention.

The fifth object of the invention indicated above may be achievedaccording to a seventh aspect of the invention, which provides a methodof producing a dielectric resonator including a dielectric ceramic and aconductor pattern embedded in the dielectric ceramic, or a dielectricfilter including the dielectric resonator, characterized in that adielectric ceramic composition according to any one of the first, secondand third aspects of the invention is co-fired with a conductive layerformed solely of Ag or of an alloy containing Ag as a major component,so as to give the dielectric ceramic and the conductor pattern.

EXAMPLES

To further clarify the concept of the present invention, some examplesof the invention will be described. It is to be understood that theinvention is not limited to the details of the illustrated examples, butmay be embodied with various alterations, modifications andimprovements, which may occur to those skilled in the art, withoutdeparting from the scope of the invention defined in the appendedclaims.

EXAMPLE 1

Initially, highly pure barium carbonate, titanium oxide, neodymiumoxide, samarium oxide, lanthanum oxide and bismuth oxide were weighed togive main ceramic compositions corresponding to samples No. 1 to No. 33and represented by the formula: xBaO·yTiO₂ ·z[(1-a)RE₂ O₃ ·aBi₂ O₃ ]wherein x, y, z, RE and a are as indicated in TABLES 1 and 2. The thusweighed materials, from which a given proportion of bismuth oxide to beadded later was excluded, were wet-blended with some pure water in apolyethylene pot mill using alumina balls. The thus obtained mixture wastaken out of the pot mill, dried, put into an alumina crucible (pot),and calcined in air for four hours at various temperatures between 900°C. and 1270° C. Then, the calcined mixture was roughly crushed, thrownback into the polyehtylene pot mill with zirconia balls, and pulverizedto achieve the average grain size of 0.4˜1.0 μm as measured by a laserdiffraction and scattering method. In this manner, calcined ceramicpowders of respective compositions were obtained.

On the other hand, highly pure zinc oxide, boric oxide and silicon oxidewere weighed in the proportion of 65% by weight of AnO, 25% by weight ofB₂ O₃ and 10% by weight of SiO₂. These compounds were thrown withalumina balls into a polyethylene pot mill, and dry-blended. The thusobtained mixture was fused in a chamotte crucible, rapidly cooled off inwater and thus vitrified. The glass obtained was thrown with aluminaballs into an alumina pot mill, and pulverized in ethanol to achieve theaverage grain size of 4 μm, to thereby provide a glass powder having acomposition (G) as indicated in TABLE 3.

Subsequently, each of the calcined ceramic powders of the testcompositions was wet-blended in pure water with the bismuth oxide to beadded later, and a predetermined amount (as indicated in TABLES 1 and 2)of the glass having the composition (G), in a polyethylene pot millusing alumina balls. At the same time, 1% by weight of polyvinyl alcohol(PVA) was added as a binder. The thus obtained mixture was then dried,passed through a sieve having openings of 355 μm, and thus granulated.In TABLES 1 and 2, Bi₂ O₃ to be added later to the main ceramiccomposition (calcined ceramic powder) should be interpreted as theproportion thereof to the total amount of Bi₂ O₃ used, while the amountof addition of the glass is represented by parts by weight per 100 partsby weight of the total amount of the calcined ceramic powder and Bi₂ O₃later added.

The thus prepared granules of each composition were formed with a pressat a surface pressure of 1t/cm², into a circular disc having a diameterof 20 mm and a thickness of 15 mm. The circular discs corresponding tothe respective compositions were fired in air for two hours at 900° C.,to thereby provide respective samples Nos. 1-33 of dielectric ceramics.These samples were ground into circular discs each having a diameter of16 mm and a thickness of 8 mm, and the dielectric properties of eachsample were measured. More specifically, the specific dielectricconstant (εr) and unloaded Q were measured according to Hakki & Colemanmethod, while the temperature coefficient (ιf) of the resonancefrequency was measured over a range from -25° C. to 75° C. Themeasurement was effected at a frequency of 2-4 GHz. The results of themeasurement are also indicated in TABLES 1 and 2.

                                      TABLE 1                                     __________________________________________________________________________                             Bi.sub.2 O.sub.2                                                                      Calcining                                                                          Average                                                                            Glass           τf                                      later added                                                                           temp.                                                                              grain size                                                                             Parts by                                                                             Q    (ppm/              No.                                                                              x  y  z  RE        a  (% by weight)                                                                         (°C.)                                                                       (μm)                                                                            Comp.                                                                             weight                                                                             εr                                                                      (3                                                                                 °C.)        __________________________________________________________________________     1 0.155                                                                            0.670                                                                            0.175                                                                            Nd        0.15                                                                             0       1250 0.6  G   2.5  68                                                                              1160 -1                  2 "  "  "  "         "  12.5    "    "    "   "    70                                                                              1010 -2                  3 0.160                                                                            "  0.170                                                                            "         "  "       "    "    "   "    74                                                                              1150 -1                  4 "  "  "  "         0.18                                                                             0       1270 0.4  "   2.0  79                                                                              1100 -8                  5 "  "  "  0.88 Nd + 0.12 La                                                                       "  "       "    0.4  "   2.0  80                                                                              1000 0                   6 0.165                                                                            "  0.165                                                                            Nd        0.15                                                                             12.5    1250 0.6  "   2.5  75                                                                              970  1                   7 0.165                                                                            0.680                                                                            0.155                                                                            "         "  "       "    "    "   "    69                                                                              1070 5                   8 0.180                                                                            0.700                                                                            0.120                                                                            0.80 Nd + 0.20 Sm                                                                       0.10                                                                             0       1270 0.5  "   2.0  66                                                                              1400 12                  9 0.145                                                                            0.660                                                                            0.195                                                                            0.90 Nd + 0.10 La                                                                       "  "       "    "    "   2.5  72                                                                              1200 3                  10 0.155                                                                            0.670                                                                            0.175                                                                            Nd        0.10                                                                             25.0    1250 0.5  "   "    71                                                                              1340 0                  11 "  "  "  "         0.15                                                                             "       "    "    "   "    74                                                                              840  -2                 12 "  "  "  "         0.20                                                                             "       "    "    "   "    77                                                                              610  -4                 13 "  "  "  0.97 Nd + 0.03 La                                                                       "  "       "    "    "   "    78                                                                              580  4                  *14                                                                              "  "  "  0.90 Nd + 0.10 Sm                                                                       "  "       "    0.4  "   0.0  36                                                                              560  --                 15 "  "  "  "         "  "       "    "    "   1.0  76                                                                              870  -4                 16 "  "  "  "         "  "       "    "    "   2.0  81                                                                              760  -5                 17 "  "  "  "         "  "       "    "    "   5.0  79                                                                              480  -3                 *18                                                                              "  "  "  "         "  "       "    "    "   6.0  77                                                                              360  -3                 19 "  "  "  "         "  50.0     900 0.6  "   2.5  47                                                                              610  2                  20 "  "  "  "         "  "       1000 0.7  "   "    47                                                                              620  2                  21 "  "  "  "         "  "       1150 "    "   "    61                                                                              500  1                  __________________________________________________________________________     *comparative example                                                           --: unmeasurable                                                        

                                      TABLE 2                                     __________________________________________________________________________                             Bi.sub.2 O.sub.2                                                                      Calcining                                                                          Average                                                                            Glass           τf                                      later added                                                                           temp.                                                                              grain size                                                                             Parts by                                                                             Q    (ppm/              No.                                                                              x  y  z  RE        a  (% by weight)                                                                         (°C.)                                                                       (μm)                                                                            Comp.                                                                             weight                                                                             εr                                                                      (3                                                                                 °C.)        __________________________________________________________________________    22 0.155                                                                            0.670                                                                            0.175                                                                            0.90 Nd + 0.10 Sm                                                                       0.20                                                                             25.0    1150 0.7  G   2.5  67                                                                              560   1                 23 "  "  "  "         "  "       1200 "    "   "    70                                                                              560  -2                 24 "  "  "  "         "  "       1250 0.8  "   "    70                                                                              580  -5                 25 "  "  "  "         "  "       1270 "    "   "    72                                                                              610  -7                 26 "  "  "  "         "  50.0    1150 1.0  "   "    57                                                                              460    1                27 "  "  "  "         "  "       "    0.8  "   "    62                                                                              491   1                 28 "  "  "  "         "  "       "    0.5  "   "    73                                                                              520   0                 29 "  "  "  "         "  25.0    1250 0.4  "   "    82                                                                              690  -3                 30 "  "  "  Nd        "   0.0    "    0.6  "   "    66                                                                              690  -3                 31 "  "  "  "         "  25.0    "    0.7  "   "    71                                                                              660  -3                 32 "  "  "  "         "  50.0    "    "    "   "    66                                                                              560  -3                 33 "  "  "  "         "  100.0   "    0.9  "   "    65                                                                              530  -3                 __________________________________________________________________________

EXAMPLE 2

Initially, highly pure barium carbonate, titanium oxide, neodymiumoxide, samarium oxide and bismuth oxide were weighed so as to give amain ceramic composition as represented by the above-indicated formula,wherein x, y, z and a are 0.145, 0.675, 0.180 and 0.20, respectively,and RE consists of 0.90 Nd and 0.10 Sm. The weighed materials, fromwhich 5% by weight of bismuth oxide was excluded, were calcined at 1170°C. in the same manner as EXAMPLE 1, and the calcined mass was pulverizedto provide a calcined ceramic powder having the average grain size of0.6 μm.

On the other hand, highly pure zinc oxide, boric oxide and silicon oxidewere weighed in respective weight ratios as indicated in TABLE 3 below,vitrified in the same manner as EXAMPLE 1, and then pulverized toprovide glass powders of respective compositions, each having theaverage grain size of 4 μm.

Subsequently, the thus obtained glass powder of each composition wasadded to the above-indicated calcined ceramic powder, along with lateradded bismuth oxide (5% by weight), and the mixture was press-formedinto a test sample in the same manner as TABLE 1, and fired in air fortwo hours at 900° C. In this manner, there were obtained samples Nos.34-52 of dielectric ceramics as indicated in TABLE 4. In TABLE 4, theamount of the glass powder is represented by parts by weight per 100parts by weight of a total amount of the calcined ceramic powder andlater added bismuth oxide.

                  TABLE 3                                                         ______________________________________                                        Glass    ZnO       B2O3      SiO2                                             composition                                                                            (k wt. %) (m wt. %) (n wt. %)                                                                             Impurities*                              ______________________________________                                        A        67        22        11                                               B        66        24        10      Al.sub.2 O.sub.3 : 2.0                   C        66        24        10      MgO: 2.0                                 D        66        24        10      SnO.sub.2 : 2.0                          E        65        27         8                                               F        65        26         9                                               G        65        25        10                                               H        65        20        15                                               I        63        29         8                                               J        63        28         9                                               K        63        27        10                                               L        63        26        11      PbO: 5.3                                 M        61        29        10                                               N        65        10        25                                               O        54        22        24                                               P        73        24         3                                               Q        40        30        30                                               R        55        35        10                                               S        45        40        15                                               ______________________________________                                         *% by weight with respect to the total amount of major components (ZnO,       B.sub.2 O.sub.3 and SiO.sub.2) of the glass composition                  

The dielectric properties of each of the fired samples Nos. 34-52 weremeasured in the same manner as EXAMPLE 1, and the results of themeasurement are also indicated in TABLE 4. As will be understood fromthis table, any of the dielectric ceramics (sintered body) obtained inthis example has a sufficiently high dielectric constant, large unloadedQ and a sufficiently small temperature coefficient of the resonancefrequency, irrespective of the relatively low firing temperature, i.e.,900° C.

                  TABLE 4                                                         ______________________________________                                        Glass                                                                                          Parts by      Q       τf                                 No.   Composition                                                                              weight    εr                                                                        (3 GHz) (ppm/°C.)                       ______________________________________                                        34    A          2.5       54  520     --                                     35    B          2.5       67  600     -4                                     36    C          2.5       67  590     -2                                     37    D          2.5       67  550     -2                                     38    E          2.5       71  550     -3                                     39    F          2.5       70  560     -3                                     40    G          2.5       68  560     -2                                     41    H          2.5       54  600     --                                     42    I          2.5       72  570     -3                                     43    J          2.5       72  580     -1                                     44    K          2.5       70  580     -2                                     45    L          2.5       63  610     -4                                     46    M          2.5       70  580     -2                                     47    N          2.5       52  550      2                                     48    O          2.5       54  580      1                                     49    P          2.5       69  610     -4                                     50    Q          2.5       73  780     -1                                     51    R          2.5       74  700     -3                                     52    S          2.5       75  650     -2                                     ______________________________________                                         --: unmeasurable                                                         

EXAMPLE 3

The calcined ceramic powder obtained in producing sample No. 7 of TABLE1 was wet-blended in an alumina pot mill using zirconia balls, withbismuth oxide to be added later (25% by weight of the total amountthereof), the glass powder of ZnO--B₂ O₃ --SiO₂ system having thecomposition (G) of TABLE 3, polyvinyl butyral, a plasticizer and apeptizing agent, within a mixed solution of toluene and isopropylalcohol.

The thus prepared mixture was degassed, and formed by a doctor bladetechnique into green tapes each having a thickness of 250 μm. Then, aconductor pattern for a 900 MHz 3-resonator bandpass filter was printedon one of the thus formed green tapes, by using an Ag paste suited forprinting. Thereafter, 12 sheets of the green tapes, including as anintermediate sheet the above-indicated one tape on which the conductorpattern was printed, were laminated at 100 kgf/cm² at 100° C. Thelaminated green tapes were cut into segments, and then fired in air fortwo hours at 900° C., to thereby provide stripline type filters. Uponmeasurement of the filter characteristics by means of a networkanalyzer, the thus obtained stripline type filters exhibited a centerfrequency of 930 MHz, and an insertion loss of 2.8 dB.

EXAMPLE 4

Initially, highly pure barium carbonate, titanium oxide, neodymium oxideand bismuth oxide were weighed so as to give a main ceramic compositionas represented by the above-indicated formula, wherein x, y, z and a are0.155, 0.670, 0.175 and 0.05, respectively. A mixture of these compoundswere calcined at 1270° C., and then pulverized to provide a calcinedceramic powder having the average grain size of 0.6 μm, in the samemanner as EXAMPLE 1.

Subsequently, the glass powder prepared according to EXAMPLE 2 andhaving the composition (I) was added in various proportions as indicatedin TABLE 5, per 100 parts by weight of the thus obtained calcinedceramic powder. Then, the mixture of the glass powder and calcinedceramic powder was press-formed into a green sheet, and then fired, inthe same manner as EXAMPLE 1.

The dielectric properties of each of fired samples No. 53 through No. 57thus obtained were measured in the same manner as EXAMPLE 1. The resultsof the measurement are indicated in TABLE 5.

                  TABLE 5                                                         ______________________________________                                                Glass (parts        Q      τf                                     No.     by weight) εr                                                                             (3 GHz)                                                                              (ppm/°C.)                           ______________________________________                                        *53     0.0        33       320    --                                         54      2.5        58       1200   3                                          55      5.0        65       1500   5                                          56      10.0       59       870    4                                          *57     15.0       52       450    3                                          ______________________________________                                         *comparative example                                                          --: unmeasurable                                                         

EXAMPLE 5

Initially, highly pure barium carbonate, titanium oxide, neodymium oxideand bismuth oxide were weighed such that x, y, z and a in theabove-indicated formula are 0.155, 0.670, 0.175 and 0.05, respectively.To the thus weighed components, there was further added alumina (Al₂ O₃)in various proportions as indicated in TABLE 6, per 100 parts by weightof the total amount of the above components. A mixture of thesecomponents and alumina was calcined at 1270° C., and then pulverized toprovide a calcined ceramic powder having the average grain size of 0.6μm, in the same manner as EXAMPLE 1.

Subsequently, the glass powder prepared according to EXAMPLE 2 andhaving the composition (I) was added to the thus obtained calcinedceramic powder, in an amount of 5 parts by weight per 100 parts byweight of the ceramic powder. Then, the mixture of the glass powder andcalcined ceramic powder was press-formed into a green sheet, and thenfired, in the same manner as EXAMPLE 1.

The dielectric properties of each of fired samples No. 58 through No. 62thus obtained were measured in the same manner as EXAMPLE 1. The resultsof the measurement are indicated in TABLE 6.

                  TABLE 6                                                         ______________________________________                                                Al2O3 (parts        Q      τf                                     No.     by weight) εr                                                                             (3 GHz)                                                                              (ppm/°C.)                           ______________________________________                                        58      0.0        65       1500   5                                          59      0.6        62       1630   2                                          60      1.2        58       1650   0                                          61      2.0        52       1380   -4                                         62      3.0        44       1030   -8                                         ______________________________________                                    

EXAMPLE 6

Initially, highly pure barium carbonate, titanium oxide, neodymiumoxide, samarium oxide, lanthanum oxide and bismuth oxide were weighed,to give test compositions corresponding to samples No. 63 through No. 73of the above-indicated formula in which x, y, z and a are respectivelydetermined as indicated in TABLE 7. To the thus weighed components,there was further added manganese oxide (MnO) in various proportions asindicated in TABLE 7, per 100 parts by weight of the total amount of theabove components. A mixture of these components and manganese oxide wascalcined at 1270° C., and then pulverized to provide a calcined ceramicpowder having the average grain size of 0.6 μm, in the same manner asEXAMPLE 1.

The dielectric properties of each of fired samples Nos. 63-73 thusobtained were measured in the same manner as EXAMPLE 1. The results ofthe measurement are indicated in TABLE 7.

                                      TABLE 7                                     __________________________________________________________________________                                MnO      Glass          Q    τf               No.                                                                              x   y  z   RE        a   (parts by weight)                                                                      Comp.                                                                             Parts by weight                                                                       εr                                                                       (3 GHz)                                                                            (ppm/°C.)     __________________________________________________________________________    63 0.165                                                                             0.670                                                                            0.165                                                                             0.95 Nd + 0.05 La                                                                       0.05                                                                              0.5      I   5.0     65 1260 11                   64 0.160                                                                             0.670                                                                            0.170                                                                             Nd        0.15                                                                              0.0      G   2.5     72 1030 5                    65 "   "  "   "         "   0.5      "   "       71 1210 5                    66 "   "  "   "         "   1.0      "   "       72 1170 3                    67 "   "  "   "         "   2.0      "   "       70 1090 1                    68 "   "  "   "         "   3.0      "   "       68  990 -1                   69 "   "  "   "         "   4.0      "   "       63  620 -1                   *70                                                                              0.155                                                                             0.675                                                                            0.170                                                                             0.80 Nd + 0.200 Sm                                                                          0.5      I   0.0     33  650 --                   71 "   "  "   "         "   "        "   1.0     72 1140 -3                   72 "   "  "   "         "   "        "   3.0     73 1080 -2                   *73                                                                              "   "  "   "         "   "        "   6.0     69  430 --                   __________________________________________________________________________     *comparative example                                                          --: unmeasurable                                                         

EXAMPLE 7

Initially, highly pure barium carbonate, titanium oxide, neodymiumoxide, samarium oxide, lanthanum oxide, alumina and manganese oxide wereweighed so as to give test compositions corresponding to samples No. 74through No. 96 and having respective values or amounts of x, y, z, RE,Al₂ O₃ and MnO as indicated in TABLE 8. The thus weighed materials werewet-blended with some pure water in a polyethylene pot mill usingalumina balls. The thus obtained mixture was taken out of the pot mill,dried, put into an alumina crucible, and calcined in air for four hoursat 1250° C. Then, the calcined mixture was roughly crushed, thrown backinto the polyethylene pot mill with zirconia balls, and pulverized toachieve the average grain size of 0.8 μm as measured by a laserdiffraction and scattering method. In this manner, calcined ceramicpowders of respective compositions were obtained.

On the other hand, highly pure zinc oxide, boric oxide and silicon oxidewere weighed to give a composition (I) indicated in TABLE 3, whichconsists of 63% by weight of ZnO, 29% by weight of B₂ O₃ and 8% byweight of SiO₂, or a composition (G) indicated in TABLE 3, whichconsists of 65% by weight of ZnO, 25% by weight of B₂ O₃ and 10% byweight of SiO₂ . These compounds were thrown with alumina balls into apolyethylene pot mill, and dry-blended. The thus obtained mixture wasfused in a chamotte crucible, rapidly cooled off in water and thusvitrified. The glass obtained was thrown with alumina balls into analumina pot mill, and pulverized in ethanol to achieve the average grainsize of 4 μm, to thereby provide a glass powder having the composition(I) or (G).

Subsequently, each of the calcined ceramic powders of the testcompositions was wet-blended in pure water with a predetermined amount(as indicated in TABLE 8) of the glass having the composition (I) or(G), in a polyethylene pot mill using zirconia balls. At the same time,1% by weight of polyvinyl alcohol (PVA) was added as a binder. The thusobtained mixture was then dried, passed through a sieve having openingsof 355 μm, and thus granulated. In TABLE 8, the amount of addition ofthe glass is represented by part by weight per 100 parts by weight ofthe amount of the calcined ceramic powder, i.e., the main ceramiccomposition.

The thus prepared granules of each composition were formed with a pressat a surface pressure of 1t/cm², into a circular disc having a diameterof 20 mm and a thickness of 15 mm. The circular discs corresponding tothe respective compositions were fired in air for two hours at 900° C.,to thereby provide respective samples Nos. 74-96 of dielectric ceramics.These samples were ground into circular discs each having a diameter of16 mm and a thickness of 8 mm, and the dielectric properties of eachsample were measured. More specifically, the specific dielectricconstant (εr) and unloaded Q were measured according to Hakki & Colemanmethod, while the temperature coefficient (τf) of the

                                      TABLE 8                                     __________________________________________________________________________                            Al.sub.2 O.sub.2                                                                    MnO                                                                     (parts                                                                              (parts by                                                                           Glass           Q    τf               No.                                                                              x   y  z   RE        by weight)                                                                          weight)                                                                             Comp.                                                                             Parts by weight                                                                       εr                                                                        (3 GHz)                                                                            (ppm/°C.)     __________________________________________________________________________    74 0.155                                                                             0.670                                                                            0.175                                                                             Nd        0.0   0.0   I   2.5     56   980 18                   75 "   "  "   "         "     "     "   6.0     65  1220 15                   76 0.125                                                                             0.695                                                                            0.180                                                                             0.80 Nd + 0.20 Sm                                                                       "     "     "   4.0     50  1300 12                   77 0.160                                                                             0.675                                                                            0.165                                                                             "         "     "     "   4.0     63  1150 16                   78 0.180                                                                             0.700                                                                            0.120                                                                             Nd        "     "     "   2.5     40  1520 38                   79 0.140                                                                             0.670                                                                            0.190                                                                             0.95 Nd + 0.05 La                                                                       "     "     "   "       50   850  6                   80 "   "  "   "         0.6   "     "   "       45  1160  1                   81 "   "  "   "         1.2   "     "   "       43  1110 -2                   82 "   "  "   "         2.0   "     "   "       40   980 -5                   83 "   "  "   "         3.0   "     "   "       34   750 -8                   84 0.125                                                                             0.695                                                                            0.180                                                                             0.90 Nd + 0.10 Sm                                                                       0.0   0.1   "   4.0     50  1450 10                   85 0.150                                                                             0.665                                                                            0.185                                                                             0.80 Nd + 0.20 Sm                                                                       "     0.5   "   4.0     62  1340 12                   86 0.180                                                                             0.700                                                                            0.120                                                                             0.95 Nd + 0.05 La                                                                       "     0.2   "   6.0     43  1700 18                   87 0.160                                                                             0.670                                                                            0.170                                                                             0.90 Nd + 0.10 Sm                                                                       "     0.0   G   5.0     65  1050 18                   88 "   "  "   "         "     2.0   "   "       63  1190 16                   89 "   "  "   "         "     3.0   "   "       60  1060 14                   90 "   "  "   "         "     4.0   "   "       54   820 12                   *91                                                                              0.145                                                                             0.675                                                                            0.180                                                                             0.80 Nd + 0.20 Sm                                                                       1.2   0.0   "   0.0     30   240 --                   92 "   "  "   "         "     "     "   3.0     46  1070 -3                   93 "   "  "   "         "     "     "   5.0     53  1390 -3                   94 "   "  "   "         "     "     "   10.0    46  1110 -4                   95 "   "  "   "         "     "     "   15.0    45   970 -4                   *96                                                                              "   "  "   "         "     "     "   20.0    43   420 -5                   __________________________________________________________________________     *comparative example                                                          --: unmeasurable   resonance frequency was measured over a range from -       25° C. to 75° C. The measurement was effected at a frequency     of 2-4 GHz. The results of the measurement are also indicated in TABLE 8.

EXAMPLE 8

The calcined ceramic powder obtained in producing sample No. 86 of TABLE8 was wet-blended in an alumina pot mill using zirconia balls, with theglass powder of the ZnO--B₂ O₃ --SiO₂ system having the composition (G)of TABLE 3, polyvinyl butyral (8 parts by weight), and suitable amountsof a plasticizer and a peptizing agent, within a mixed solution oftoluene and isopropyl alcohol.

The thus prepared mixture was degassed, and formed by a doctor bladetechnique into green tapes each having a thickness of 250 μm. Then, aconductor pattern for a 900 MHz 3-resonator bandpass filter was printedon one of the thus formed green tapes, by using an Ag paste suited forprinting. Thereafter, 20 sheets of the green tapes, including as anintermediate sheet the above-indicated one tape on which the conductorpattern was printed, were laminated at 100 kgf/cm² at 100° C. Thelaminated green tapes were cut into segments, and then fired in air fortwo hours at 900° C., to thereby provide stripline type filters. Uponmeasurement of the filter characteristics by means of a networkanalyzer, the thus obtained stripline type filters exhibited a centerfrequency of 930 MHz, and an insertion loss of 2.8 dB.

EXAMPLE 9

The main ceramic composition (in the form of calcined ceramic powder) asused for sample No. 88 of TABLE 8 was prepared, while glass powdershaving the compositions (N), (O), (P), (Q), (R) and (S) as indicated inTABLE 3 were prepared.

Then, the prepared calcined ceramic powder was wet-blended in pure waterwith the glass powder of each composition, in a polyethylene pot millusing alumina balls. The glass powder was contained in an amount of 5parts by weight per 100 parts by weight of the calcined ceramic powder(main ceramic composition). The thus obtained mixture was granulated,and the granules of each composition was press-formed into a circulardisc, in the same manner as EXAMPLE 7. The circular discs correspondingto the respective compositions were fired in air for two hours at 900°C., to thereby provide samples No. 97 through No. 102 of dielectricceramics as indicated in TABLE 9. The dielectric properties of thesesamples were measured, and the results of the measurement are indicatedin TABLE 9.

                  TABLE 9                                                         ______________________________________                                        Glass                                                                                          Parts by      Q      τf                                  No.  Composition weight   εr                                                                         (3 GHz)                                                                              (ppm/°C.)                        ______________________________________                                         97  N           5.0      49   1060   20                                       98  O           5.0      54   1210   19                                       99  P           5.0      63   1050   18                                      100  Q           5.0      66   1250   17                                      101  R           5.0      69   1200   15                                      102  S           5.0      70   1170   16                                      ______________________________________                                    

EXAMPLE 10

Initially, highly pure barium carbonate, strontium oxide, calcium oxide,titanium oxide, neodymium oxide, samarium oxide, lanthanum oxide andbismuth oxide were weighed to give main ceramic compositionscorresponding to samples No. 103 through No. 117 and represented by theformula: x[(1-c-d)BaO·cSrO·dCaO]·yTiO₂ ·z[(1-e)RE₂ O₃ ·eBi₂ O₃ ] whereinx, y, z, RE, c, d and e are as indicated in TABLE 10. The thus weighedmaterials were wet-blended with pure water in a polyethylene pot millusing zirconia balls. The thus obtained mixture was taken out of the potmill, dried, put into an alumina crucible, and calcined in air for fourhours at 1250° C. Then, the calcined mixture was roughly crushed, thrownback into the polyethylene pot mill with zirconia balls, and pulverizedto achieve the average grain size of 0.8 μm as measured by a laserdiffraction and scattering method. In this manner, calcined ceramicpowders of respective compositions were obtained.

On the other hand, highly pure zinc oxide, boric oxide and silicon oxidewere weighed to give the composition (I) indicated in TABLE 3, whichconsists of 63% by weight of ZnO, 29% by weight of B₂ O₃ and 8% byweight of SiO₂, or the composition (G) indicated in TABLE 3, whichconsists of 65% by weight of ZnO, 25% by weight of B₂ O₃ and 10% byweight of SiO₂. These compounds were thrown with alumina balls into apolyethylene pot mill, and dry-blended. The thus obtained mixture wasfused in a chamotte crucible, rapidly cooled off in water and thusvitrified. The glass obtained was thrown with alumina balls into analumina pot mill, and pulverized in ethanol to achieve the average grainsize of 4 μm, to thereby provide a glass powder having the composition(I) or (G).

Subsequently, each of the calcined ceramic powders of the testcompositions was wet-blended in pure water with a predetermined amount(as indicated in TABLE 10) of the glass having the composition (I) or(G), in a polyethylene pot mill using alumina balls. At the same time,1% by weight of polyvinyl alcohol (PVA) was added as a binder. The thusobtained mixture was then dried, passed through a sieve having openingsof 355 μm, and thus granulated.

The thus prepared granules of each composition were formed with a pressat a surface pressure of 1 t/cm², into a circular disc having a diameterof 20 mm and a thickness of 15 mm. The circular discs corresponding tothe respective compositions were fired in air for two hours at 900° C.,to thereby provide respective samples Nos. 103-117 of dielectricceramics. These samples were ground into circular discs each having adiameter of 16 mm and a thickness of 8 mm, and the dielectric propertiesof each sample were measured. More specifically, the specific dielectricconstant (εr) and unloaded Q were measured according to Hakki & Colemanmethod, while the temperature coefficient (τf) of the resonancefrequency was measured over a range from -25° C. to 75° C. Themeasurement was effected at a frequency of 2-4 GHz. The results of themeasurement are also indicated in TABLE 10.

                                      TABLE 10                                    __________________________________________________________________________                                    Glass         Q    τf                     No. x  y  z  RE        c  d  e  Comp.                                                                             Parts by weight                                                                       εr                                                                      (3 GHz)                                                                            (ppm/°C.)           __________________________________________________________________________    103 0.125                                                                            0.695                                                                            0.180                                                                            0.90 Nd + 0.10 Sm                                                                       0.25                                                                             0.0                                                                              0.0                                                                              I   6.0     55                                                                              1350 12                         104 0.150                                                                            0.670                                                                            0.180                                                                            0.80 Nd + 0.20 Sm                                                                       0.05                                                                             "  "  "   6.0     65                                                                              1250 13                         105 0.180                                                                            0.700                                                                            0.120                                                                            Nd        0.35                                                                             "  "  "   7.0     45                                                                              1540 19                         106 0.160                                                                            0.675                                                                            0.165                                                                            "         0.0                                                                              0.05                                                                             "  "   4.0     63                                                                              1180 18                         107 0.155                                                                            0.670                                                                            0.175                                                                            0.95 Nd + 0.05 La                                                                       0.10                                                                             0.00                                                                             0.10                                                                             "   3.0     70                                                                              1370  0                         108 "  0.675                                                                            0.170                                                                            Nd        0.00                                                                             "  0.15                                                                             G   2.5     69                                                                              1050  6                         109 "  "  "  "         0.20                                                                             "  "  "   "       67                                                                              1140  2                         110 "  "  "  "         0.40                                                                             "  "  "   "       64                                                                              1030  0                         111 "  "  "  "         0.50                                                                             "  "  "   "       57                                                                               840 -3                         112 "  "  "  "         0.00                                                                             0.20                                                                             "  "   "       70                                                                               960  9                         113 "  "  "  "         "  0.30                                                                             "  "   "       70                                                                               470 13                         *114                                                                              "  0.660                                                                            0.185                                                                            0.90 Nd + 0.10 La                                                                       0.10                                                                             0.05                                                                             0.25                                                                             I   0.0     36                                                                               540 --                         115 "  "  "  "         "  "  "  "   1.0     69                                                                              1130 -4                         116 "  "  "  "         "  "  "  "   3.0     74                                                                              1010 -3                         *117                                                                              "  "  "  "         "  "  "  "   6.0     71                                                                               360 --                         __________________________________________________________________________     *comparative example                                                          --: unmeasurable                                                         

EXAMPLE 11

The calcined ceramic powder obtained in producing sample No. 109 ofTABLE 10 was wet-blended in an alumina pot mill using zirconia balls,with the glass powder of the ZnO--B₂ O₃ --SiO₂ system having thecomposition (G) of TABLE 3, polyvinyl butyral (8 parts by weight), andsuitable amounts of a plasticizer and a peptizing agent, within a mixedsolution of toluene and isopropyl alcohol.

The thus prepared mixture was degassed, and formed by a doctor bladetechnique into green tapes each having a thickness of 250 μm. Then, aconductor pattern for a 900 MHz 3-resonator bandpass filter was printedon one of the thus formed green tapes, by using an Ag paste suited forprinting. Thereafter, 12 sheets of the green tapes, including as anintermediate sheet the above-indicated one tape on which the conductorpattern was printed, were laminated at 100 kgf/cm² at 100° C. Thelaminated green tapes were cut into segments, and then fired in air fortwo hours at 900° C., to thereby provide stripline type filters. Uponmeasurement of the filter characteristics by means of a networkanalyzer, the thus obtained stripline type filters exhibited a centerfrequency of 930 MHz, and an insertion loss of 2.8 dB.

EXAMPLE 12

The main ceramic composition (in the form of calcined ceramic powder) asused for sample No. 109 of TABLE 10 was prepared, while glass powdershaving the compositions (N), (O), (P), (Q), (R) and (S) as indicated inTABLE 3 were prepared.

Then, the prepared calcined ceramic powder was wet-blended in pure waterwith the glass powder of each composition, in a polyethylene pot millusing alumina balls. The glass powder was contained in an amount of 2.5parts by weight per 100 parts by weight of the calcined ceramic powder(main ceramic composition). The thus obtained mixture was granulated,and the granules of each composition was press-formed into a circulardisc, in the same manner as EXAMPLE 10. The circular discs correspondingto the respective compositions were fired in air for two hours at 900°C., to thereby provide samples No. 118 through No. 123 of dielectricceramics as indicated in TABLE 11. The dielectric properties of thesesamples were measured, and the results of the measurement are indicatedin TABLE 11.

                  TABLE 11                                                        ______________________________________                                        Glass                                                                                          Parts by      Q      τf                                  No.  Composition weight   εr                                                                         (3 GHz)                                                                              (ppm/°C.)                        ______________________________________                                        118  N           2.5      52   1000   4                                       119  O           2.5      57   1150   4                                       120  P           2.5      68   1120   1                                       121  Q           2.5      71   1200   2                                       122  R           2.5      73   1080   1                                       123  S           2.5      73   1020   -1                                      ______________________________________                                    

The dielectric ceramic compositions prepared according to the aboveEXAMPLES 1-12 can be fired or sintered at 1000° C. or lower, preferably,at around 900° C. The dielectric ceramics obtained from the presentdielectric compositions are advantageously used for producing adielectric filter, such as a stripline type filter, which incorporatesinternal conductive layers formed solely of Ag having a relatively lowconductivity resistance, or of alloys containing Ag as a majorcomponent. Further, the dielectric ceramics exhibit a sufficiently highspecific dielectric constant, a sufficiently large unloaded Q and asignificantly reduced temperature coefficient of the resonancefrequency.

What is claimed is:
 1. A dielectric ceramic composition which consistsessentially of: a main ceramic composition containing barium oxide,titanium oxide, rare earth oxide and bismuth oxide as major components,which composition is represented by xBaO·yTiO₂ ·z[(1-a)RE₂ O₃ ·aBi₂ O₃ ]where RE represents at least one rare earth metal, 0.10≦x≦0.20,0.60≦y≦0.75, 0.10≦z≦0.25, x+y+z=1 and 0<a≦0.3; and a glass compositioncomposed of ZnO, B₂ O₃ and SiO₂, which is represented by k(wt.%)ZnO·m(wt. %)B₂ O₃ ·n(wt. %)SiO₂ where 30≦k≦85, 5≦m≦50, 2≦n≦40,k+m+n=100, said glass composition being contained in an amount of atleast 0.1 part by weight, but not more than (18-62.5a) parts by weight(where 0<a=0.2) or not more than 5.5 parts by weight (where 0.2<a≦0.3),per 100 parts by weight of said main ceramic composition.
 2. Adielectric ceramic composition according to claim 1, wherein said mainceramic composition further contains not more than 2.5 parts by weightof alumina per 100 parts by weight of a total amount of said majorcomponents.
 3. A dielectric ceramic composition according to claim 2,wherein said alumina is contained in said main ceramic composition, inan amount of not more than 2 parts by weight per 100 parts by weight ofthe total amount of said major components.
 4. A dielectric ceramiccomposition according to claim 1, wherein said main ceramic compositionfurther contains not more than 3 parts by weight of manganese oxide per100 parts by weight of a total amount of said major components.
 5. Adielectric ceramic composition according to claim 4, wherein saidmanganese oxide is contained in said main ceramic composition, in anamount of not more than 2 parts by weight per 100 parts by weight of thetotal amount of said major components.
 6. A dielectric ceramiccomposition according to claim 1, wherein said at least one rare earthmetal as represented by RE is selected from the group consisting of Nd,Sm, La, Ce and Pr.
 7. A dielectric ceramic composition according toclaim 6, wherein said at least one rare earth metal (RE) consists of Nd.8. A dielectric ceramic composition according to claim 6, wherein saidat least one rare earth metal (RE) is a combination of Nd and Sm and/orLa.
 9. A dielectric ceramic composition according to claim 1, wherein atleast 5 mole % (1≧0.05) of RE₂ O₃ is substituted by Bi₂ O₃.
 10. Adielectric ceramic composition according to claim 1, wherein said glasscomposition consists of 40-75 wt. % of said ZnO (40≦k≦75), 10-40 wt. %of said B₂ O₃ (10≦m≦40), and 5-30 et. % of said SiO₂ (5≦n≦30).